In line with the rapid development of mobile communication and Internet technologies and the increasing demands for various services subscribers, the recent mobile communication services have been advanced to provide not only the existing voice call service, but also a high-speed packet data service capable of transmitting E-mails, still images and massive digital data over the mobile terminals. Accordingly, mobile communication systems have been evolving from the voice-based Circuit Switched (CS) domain into the packet-based Packet Switched (PS) domain.
A Core Network (CN) providing the multimedia services over the Internet is making progress from the existing circuit-switched CN to a packet-switched CN, evolving into an Internet Protocol (IP)-based CN. A communication system providing IP multimedia services to subscribers over the IP-based CN is called an IP Multimedia Subsystem (IMS). In 3rd Generation Partnership Project (3GPP)/3rd Generation Partnership Project 2 (3GPP2), many studies are underway to seamlessly provide ALL IP service over the IMS.